Estimating plume degradation rates in aquifers: Effect of propagating measurement and methodological errors

Field estimates of plume degradation rates λ [T −1 ] in aquifers provide a basis for assessing the possible impact of (toxic) organic pollutants on downstream environments; however, difficulties with measurement and methodology mean that estimated site‐specific rates potentially involve considerable uncertainty. Here, we specifically show that if mass flow or average concentration measurements are associated with errors of ∼20% (or even less), the errors may in many cases propagate, magnify, and cause order‐of‐magnitude errors in estimates of λ . We also investigate uncertainties in the integral pumping test method, in which average concentrations are determined based on concentrations measured from pumping wells. In this method, the small‐scale variability that may bias the results of point measurements can be favorably averaged out when pumping; however, the novelty of the approach means that questions remain regarding its application. For example, the magnitude of methodological errors, such as those associated with the assumption of constant concentration along the flow direction within the extent of the well capture zone, remain poorly understood. This assumption can be violated by the biodegradation of contaminants, thereby leading to a bias in subsequent interpretations. We provide an analytical expression from which the prediction error due to attenuating concentrations can be evaluated and show its dependence on the degradation rate, the degradation function, and the extent of the well capture zone. Even for considerable first‐order degradation, the mass flow error generally remains small and is not magnified in estimates of λ .